I. Field of the Invention
The present invention relates to a steerable antenna system to facilitate communication between a mobile transceiver and a central station via a satellite. In particular, the present invention relates to a small aperture antenna that adjusts in azimuth and elevation to more efficiently acquire a geosynchronous satellite for communication with a central transmission facility.
II. Description of the Related Art
Mobile communication systems are utilized by commercial trucking companies to locate, identify and ascertain the status of their vehicles. Mobile communications systems are also used to send information, and receive information and information requests from the operator of the vehicles.
Such mobile communication systems often operate by sending signals from a home base or hub, also referred to as a central or fixed station, to the truck via a satellite. The truck typically has an antenna mounted on an upper surface for receiving information from the hub via the satellite. In some systems, a transceiver located in the truck operates via the antenna to send information back to the hub via the satellite.
In order for the small aperture antenna to acquire a geosynchronous satellite and maintain contact with the hub via the satellite, the antenna must be configured to adjust its position. Typically, these antennas are configured to sweep through an arc of rotation to acquire the satellite. For example, during initial acquisition, such as when the vehicle first engages the system after an off period, the antenna has no way of knowing where the satellite is located. Also, during use, when a truck turns a corner, the relative position of the antenna to the satellite changes, and the antenna must be able to maintain contact with the hub and satellite. In both cases, the antenna is configured to adjust its azimuthal position to acquire and track the satellite during movement of the vehicle.
One problem with conventional small aperture antennas is that even though they are rotatable, they are often fixed in elevational position. As the vehicle moves a substantial distance away from the orbital track of the satellite, the satellite moves lower on the horizon relative to the antenna. In this case, a conventional antenna cannot adjust its elevational position to maintain contact with the satellite. To accommodate this problem, vehicles are often equipped with antennas having a fixed high or low elevation, called a "look angle", depending on where the vehicles are generally driven in relation to the satellite's orbital track. For example, if the satellite is in geosynchronous orbit, it is generally fixed over a certain position on the earth and orbits at the same speed as the Earth's rotation along a predetermined longitude. In this example, if the satellite is in geosynchronous orbit along a longitude in the center of the United States, vehicles that are typically driven in higher latitudes, e.g., Canada, would have an antenna with a lower look angle, vehicles typically driven at or near the center of the U.S. would have an antenna with a higher look angle, and vehicles driven in lower latitudes, e.g., Mexico, would have antennas with a very high look angle. It is apparent that, in conventional, azimuth-only tracking systems, a single small aperture antenna cannot be used globally.
Another problem with conventional antennas is that to change the elevation, an additional power source may be needed. For example, a conventional gimbal system exists that concurrently adjusts azimuth and elevation of an antenna. However, this system uses a separate motor for each degree of freedom. The second motor is disposed on the antenna so that it rotates with the antenna when the azimuth is adjusted. The additional weight of this second motor requires that a large motor be used to rotate the assembly in azimuth.
What is needed is an antenna that can automatically adjust both azimuth and elevation so that it can be used on a vehicle in many different locations in the world. Further, what is needed is a cost-efficient and lightweight system to automatically adjust azimuth and elevation of an antenna. Still further, what is needed is an antenna that uses the same motor to adjust both azimuth and elevation of the antenna.